Frequency selective system



W. P. MASON FREQUENCY SELECTIVE SYSTEM Oct. 17, 1950 Filed Aug. 15. 1947F/GZ.

/N 1/5 N TOR w. P. MASON X AT TOPNEV' Patented Oct. 17, 1950 FREQUENCYSELECTIVE SYSTEM Warren P. Mason, West Orange, N. J assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application August 15, 1947, Serial No. 768,835

Claims.

This invention relates to Wave transmission and more particularly to afrequency selective system for separating radiant energy into individualchannels on a frequency basis or for combining a number of differentbands of frequencies.

The principal object of the invention is to direct multifrequencyelectromagnetic waves into separate channels according to frequency orto combine a number of'separate bands into'a single channel.

Another object is to increase the number of channels into whichmultifrequency guided electromagnetic waves may be separated on thebasis of frequency 01' from which they may be combined.

A further object is to decrease the size and the cost of a multichannelsystem for separating or combining guided electromagnetic waves ofdifferent frequencies.

Another object is to increase the directivity, as a function offrequency, in a directional microwave radiator.

In wave transmission systems employing coaxial lines or wave guides; itis often required to separate inultifrequency electromagnetic Waves intoindividual channels according to frequency or to combine a number ofbands into a single channel. A difficulty encountered in such a system,when aparallel arrangement of channel filters is used, is that all ofthe other filters must have a high impedance over the band of the oneunder consideration. Tl is difiiculty may be overcome in a twoorthreemhannel system, if the bands are far enough apart, by employingcoaxial or Wave guide filters with properly designed reactance-annullingstubs. However, this soluticn is unsatisfactory for a large number ofchannels vith bands close together.

The frequency selective system in accordance with the present inventionovercomes these limitations by employing a directional radiator which isthe electromechanical equivalent of an optical prism. The may bedesigned to separate or combine successfully a dozen or more closelyspaced bands of frequencies.

In its preferred form the directional radiator comprises a section oftransmission line having an 11C10Sillg outer sheath, such as a waveguide or a coaxial line, terminated at one end in its characteristicimpedance. The sheath is provided with a number of radiating holesspaced apart an integral number of wavelengths at some frequency ,m andthe section of line is coiled into a tight helix so that the holes areadjacent to each other. At-the frequency fm the radiation from all ofthe holes is in phase and there is produced a beam of radiant energywhich is normal to the axis of the helix at the central hole; As thefrequency is changedfrom fm the direction of the beam shifts so that itmakes an increasing angle with the normal;- For frequencies above fm thebeam shifts to one side and for frequencies below fm it shifts tothe-other side of the normal. The radiator may be made more directive increasing the number of holes and by progressively decreasing their areasfrom the central to the end holes.

If the axis of the helix is a straight'line the directional radiator isfocussed on infinity. Such a radiator is useful, for example, inscanning devices of the type employed in object locating and similarsystems. Compared with a radiator using an uncoilcd line it has theconsiderable advantage that its directivity may be increased toanydesired extent simply by increasing the "integral number ofwavelengths between the radiating holes, thereby increasing the angularswing of the beam for a given change in frequency. f

To provide a frequency selective system; radi ant energy pi k-up or feeddevices are located at the points where the different bands arefocussed; Therefore, in order to make the physical size of the filterpracticable, the directional radiator is modified so that the bandswil.have finite focal points. This is accomplished by transversely displacing the coils of the helix so that the radiating holes lie onfthearc of a circle. Then Waves of the'frequency fm will focus at the centerof th s circle and frequencies above or below far will come to focus atother points lying on an are which passes through the center. Thepick-up or.feed devices for the different channels may, forexame ple, beopen-ended transmission lines of the type used in the radiator. Thewidth of the open end of the line determines the width of the band offrequencies for each channel. v

To prevent undesired radiation and improve the operation of the systemthe radiant energy is preferably confined between two parallel plateswhich extend between the radiator and the cuts ing, in which likereference characters refer to similar or corresponding parts and inwhich:

Fig. l is a perspective view of a multichannel frequency selectivesystem in accordance with the invention; and

Fig. 2 is a schematic sectional view of the system.

In the embodiment shown, the system comprises a directional radiator 3,nine transmission lines 4 to l2, and an interconnecting chamber l3.Multifrequency electromagnetic waves are introduced or withdrawn at oneend of the radiator 3, as indicated by the double-pointed arrow I4, andthe separate frequency bands are taken off or fed in through thechannels 4 to [2. The system may be used either to separate or tocombine a number of bands of frequencies.

The directional radiator 3 comprises a section of rectangular wave guidel5 having a series of thirteen circular holes such as [6, ll, I8 in oneof its side walls. These holes have a uniform spacing between centers,as measured along the axis of the guide l5, equal to an integral number,greater than unity, of wavelengths within the guide at some frequency fmwhich is preferably the geometric mean of the multifrequency waves. Thesection I5 is coiled into a helix in such a way that the centers of allof the'holes l6, ll, [8 fall in a plane through the axis of the helix.The coils of the helix are packed tightly together to reduce thedistance D between the centers of adjacent holes, such as l6 and I1.Also, in order to reduce this distance the holes are preferably made inone of the narrower sides of the guide [5. The distance D preferablydoes not exceed a wavelength in the surrounding medium, so that only asingle primary beam will be radiated. The guide I5 is terminated in animpedance I 9, indicated in Fig. 2, which matches its characteristicimpedance over the operating range and absorbs substantiall all of theenergy that reaches it. This impedance may, for example, be provided bya resistive film placed across the guide 15. The amount of energyradiated by each hole is small compared to that within the guide 15 atthat point, and the total amount radiated from all the holes ispreferably not more than that absorbed in the end termination IS. Theselectivity of the system may be increased by increasing the number ofradiating fm are fed into the left end of the radiator 3 the radiationfrom all of the holes will be in phase and the resulting beam will be ina direction normal to the axis of the helix at the central hole [6. Forhigher frequencies the beam will swing to the right and for lowerfrequencies it will swing to the left. If the holes lie in a straightline these beams will come to a focus only at an infinite distance. Sucha radiator is useful in scanning devices and has the advantage that thechange in the direction of the beam resulting from a given change infrequency may be increased to any desired extent by increasing theintegral number of wavelengths between adjacent holes, as measured alongthe axis of the guide l5. In a practical case this number may, forexample, be of the order of ten or more.

When the radiator 3 is to be used in a filtering system, it is desirablethat the beam have a finite focal point. be done by transverselydisplacing adjacent coils As shown in Figs. 1 and 2 this may of thehelix so that the centers of the holes all fall on the arc of a circle.As shown in Fig. 2 this circle is centered at the point C and has aradius R. Therefore, at the frequency fm the radiated energy from all ofthe holes will be in phase and since all of the holes are equidistantfrom the center C the energy will be focused at this point. For anyother point in space the waves from the different holes will be out ofphase and cancellation will occur. For other frequencies, however, therewill be a progressive phase shift from hole to hole and waves of thesefrequencies will be focused at other points. Thus, waves of a certainfrequency lower than fm will be focussed at point A to the left of C andthose of a certain frequency higher than ,fm will be focussed at a pointB to the right of C. All of these points A, B and C fall on a smootharc. Each of the focal points, such as B, may be determined by settingthe phase shift in the medium plus the phase shift in the guide I5 equalfor the waves radiated from the two end holes l8 and the central holel6.

As shown, the pick-up devices are the openended rectangular wave guides4 to l2 directed toward the radiator 3. The end of each guide, such asH], is centered at the focal point, such as B, for the mid-bandfrequency of the band carried by that channel. The width w of the openend determines the width of the band for the channel.

The propagation medium between the radiator 3 and the channels 4 to I2is preferably enclosed in a chamber l3 to prevent undesired radiation.The chamber l3 comprises two fan-shaped parallel plates 2! and 22extending from the radiator 3 to the ends of the guides 4 to [2.Portions of the plates 2| and 22 are broken away in Fig. 1 to expose theradiating holes I6, 11, I B. The sides of the plates 2| and 22 areconnected by the plates 23 and 24 and the spaces between the ends of theguides are closed by the plates 25. In order to prevent undesiredreflections of the radiant energ within the connecting chamber l3 theinner surfaces of the side plates 23, 2A and 25 are preferably coatedwith a material 26 which will match the propagation medium in impedance.The material 26 may, for example, be a thin film of platinum. If themedium is air, which. has an impedance of about 377 ohms per squarecentimeter, the platinum film is made approximately l0 centimetersthick.

It is to be understood that the frequency selective system described maybe used to separate a number of bands of frequencies by feeding themultifrequency electromagnetic Waves into the radiator 3 and taking offthe segregated bands through the channels 4 to l2 or it may be used tocombine a number of different bands introduced through the channels 4 to[2 and taken off at the end of the guide l5.

What is claimed is:

l. A frequency selective system for multifrequency electromagnetic wavescomprising a guide for said waves and a device for withdrawing orintroducing waves of a selected frequency, said guide having a pluralityof apertures in a wall thereof, said apertures having a uniform spacingequal to an integral number, greater than unity, of wavelengths in saidguide at an operating frequency, said guide being formed into a helixwith adjacent coils transversely displaced so that said apertures areadjacently arrayed in an are whereby waves of different frequencies whenfed into the end of said guide are focussed at different focal points,and said device being-located at one of said focal points. 1 v

2. A frequency selectivesystem in accordance with claim 1 which includesa. device for withdrawing or introducing waves of another selectedfrequency located at another of said focal points.

3. A frequency selective system in accordance with claim 1 whichincludes a chamber enclosing the propagation medium between saidapertures and said device.

4, A frequency selective system in accordance with claim 3 in which saidchamber comprises a pair of parallel plates extending from said helix tosaid device.

5. A frequency selective system in accordance with claim 4 in which saidchamber includes side walls connecting the sides of said plates and saidside walls are lined with a material which provides an impedance matchfor the enclosed propagation medium. i 6. A frequency selective systemfor multifrequency electromagnetic waves comprising a directionalradiator at one end of which said waves are introduced or withdrawn; aplurality of waveguides with open ends directed toward said radiator,and a chamber enclosing the propagation medium between said radiator andsaid guides, said radiator comprising a section of guide having a seriesof radiating holes in a wall thereof,

said holes having a uniform spacing equal to an integral number, greaterthan unity, of wavelengths in said apertured guide at an operatingfrequency, said apertured guide being terminated at one end in itscharacteristic'impedance over the operating range and being coiled intoa tight 6 7. A directive device for radiating or picking up radiantenergy of different frequencies at correspondingly different anglescomprising a section of guide at one end of which said energy isintroduced or withdrawn, said guide being terminated at its other end inan impedance which absorbs substantially all of the energy that reachesit, having a series of small holes in a wall thereof with uniformspacing equal to an integral number, greater than unity, of wavelengthsin said guide at an operating frequency and being coiled into a tighthelix so that the centers of said holes fall in a plane through the axisof said helix.

8. A directive device in accordance with claim 7 in'which said frequencyf is approximately the geometric mean of the range of operatingfrequencies.

helix with adjacent coils transversely'displaced plates extending-fromsaid radiator to said openended guides, and the centers of the openends' of said guidesbeing located, respectively, atdifferent ones ofsaid focal points. 7

9. A directive device in accordance with claim '7 in which thecross-sectional areas of said holes progressively decrease in size' fromthe center of said series to each end hole.

10. A directional device in accordance with claim 7 in which saidintegral number is at least ten.

WARREN P. MASON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Australia Nov. 19, 1942

